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Astron. Astrophys. 320, L37-L40 (1997)

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1. Introduction

The Transient X-ray source GRS1915+105 was discovered with the WATCH instrument onboard the GRANAT Observatory in 1992 (Castro-Tirado et al. 1994 ). Hard X-ray studies in 20-100 keV band have shown erratic intensity variations on time scales of days and months (Foster et al. 1996 ; Sazonov et al. 1994 ). The source has been identified with a superluminal radio source (Rodriguez & Mirabel 1993 ) which undergoes frequent flaring on a variety of time scales. Based on its peculiar radio characteristics and superluminal motion, it has been termed as a micro-quasar (Mirabel & Rodriguez 1994 ). An infrared source with a jet at the same position angle as seen in the radio counterpart, has also been identified with this object (Sams et al. 1996 ). Based on its X-ray luminosity which greatly exceeds the Eddington limit and radio characteristics which are similar to those of the radio loud quasars, it has been suggested to be a black hole. We have observed this source with the Indian X-ray Astronomy Experiment (IXAE) and have detected erratic intensity variations on time scale of 0.1s - 10s. Strong Quasi Periodic Oscillations (QPOs) in a frequency range of 0.62 to 0.82 Hz were also detected unambiguously (Agrawal et al. 1996b ). In this letter we discuss the possible emission region and mechanism of the observed quasi-periodic oscillations. The strong and erratic intensity variations are identical to the variations seen in other black-hole candidate X-ray sources. The QPOs seen in GRS 1915+105 are compared with the same in other black hole candidates and neutron star binaries.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998